For “Phase Two” of the Marvel Cinematic Universe, where Marvel Studios will build towards the highly anticipated sequel to the record-breaking Avengers event film, the young Disney-owned production house is taking a big risk in going all-out sci-fi with the obscure property known as Guardians of the Galaxy.

Guardians represents the last chapter of Phase Two before The Avengers: Age of Ultron kicks off summer 2015 and to help make the James Gunn-directed project more marketable, Marvel isn’t shying away after going after A-List Hollywood talent.

Of the five core heroes of the Guardians of the Galaxy film, the three non-animated characters are being played by Chris Pratt, Zoe Saldana and Dave Bautista, with supporting roles filled in by Oscar winner Benicio Del Toro, Oscar nominees Djimon Hounsou, Glenn Close and John C. Reilly, and popular TV stars Lee Pace, Michael Rooker and Karen Gillan. Most of the group have already begun shooting in London.

As for the two unlikeliest of heroes – the walking, talking alien tree known as Groot, and his intelligent heavy weapons expert pal Rocket Raccoon (a talking raccoon) – Marvel is closing in on officially signing and announcing who will be voicing the animated members of the Guardians of the Galaxy.

THR confirms via their sources that all of these are indeed the case, with Cooper and Olsen both in negotiations with Marvel Studios for the respective reported roles. If the casting pans out with the above talent, that leaves a few key roles left to be filmed in both projects, namely Thanos, Quicksilver and Ultron.

Ultron is the robotic villain of The Avengers 2 and there’s no indication yet on the actor or voice actor Whedon is looking for. For all we know, Paul Bettany (the voice of Tony Stark’s assistant Jarvis in all three Iron Man films and The Avengers) could become Ultron. That’s just one theory we have. As for Quicksilver, Aaron Taylor-Johnson has been in talks with Whedon and the studio for quite some time to determine what direction to take the character and performance, and it sounds like he’s an unofficial lock.

Big bad Thanos on the other hand, remains a mystery. The character was first introduced during the mysterious mid-credits scene of The Avengers (image above), much to the confusion of moviegoers unfamiliar with Marvel Comics. But for fans, the intro set the stage for the future of the franchise and the character will make a return in Guardians of the Galaxy, with several of the villains already confirmed to be working for him (Djimon Hounsou & Lee Pace’s characters). Many believed Vin Diesel was in talks with Marvel for that role, but there’s been little to no news on the casting front when it comes to The Mad Titan.

Thor: The Dark World on November 8, 2013, Captain America: The Winter Soldier on April 4, 2014, Guardians of the Galaxy on August 1, 2014, The Avengers: Age of Ultron on May 1, 2015, Ant-Man on November 6, 2015, and unannounced films for May 6 2016, July 8 2016 and May 5 2017.